1. Field of the Invention
This invention relates to polypropylene resin compositions for molding which have well-balanced mechanical strength properties and good moldability and which are capable of giving molded products having a good appearance.
2. Background Art
Heretofore, reinforced polypropylene resin compositions compounded with fillers such as talc and various whiskers or with a variety of rubbers have been widely used, by taking advantage of their excellent mechanical strength and moldability, as materials for a variety of molded products in the field of industrial parts, for example, parts of automobiles such as a bumper, an interior panel, a fan shroud and a glove compartment, and parts of household electric appliances such as a TV case, a VTR case and a cover for a washing machine.
Under such circumstances, improvements in the compositions have been made by dividing a filler into fine particles, by incorporating an ethylene--propylene copolymer rubber component, or by adding a dispersing agent such as a metallic salt or a wax when well-balanced mechanical strength properties, good moldability and a good appearance of molded products are particularly demanded.
However, each of the above manners for the improvements has problems: when a filler is divided into fine particles, the particles cause secondary flocculation, resulting in deterioration in the strength and the appearance of molded products; when a rubber component is incorporated, not only the production cost increases but also the rigidity of molded products decreases; and when a conventional dispersing agent is added, a metal mold is stained with the agent.
Specifically, the balance of mechanical strength properties between rigidity (heat resistance) and impact strength is poor, flow marks (wavy flow pattern) appear, for example, on an injection-molded product, and a mold used is stained. In addition, since the moldability (fluidity) is poor, the degree of freedom of the design of the above-described various molded products becomes low and improvement in the productivity of the molded products tends to be obstructed.
It is therefore a primary object of the present invention to solve the problems in the prior art and provide a polypropylene resin composition which has well-balanced mechanical strength properties and good moldability, and can give molded products of good appearance.